This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Keeping station in drifting pack ice is a challenging task for an offshore platform. Bottom-founded platforms have been successfully developed for shallower water. In deeper water (notionally in water depth of 75 m or greater), however, bottom-founded platforms become impractical, and floating platforms need to be employed. Such floating platforms may keep station with the help of a mooring system consisting of several mooring lines made of steel (wire or chain) or synthetic materials. Drifting pack ice impacting the floating platform produces loads in the mooring lines. Such loads can become very high when the ice conditions are severe, leading to breakage of the lines.
A ship-shape vessel is attractive as a floating platform in drifting pack ice because: it has a large deck area, it has a large under-deck volume, and ice loads on it from drifting pack ice are relatively low when the vessel is aligned with the ice drift direction.
However, if the ice drift direction changes, a ship-shape vessel could be impacted by the drifting ice on the beam, resulting in significantly higher ice loads than when aligned with the ice drift direction. Such ice loads may exceed the capacity of even the strongest mooring systems that have been designed to date.
If the mooring system is attached to a turret about which the vessel may rotate in the ice, the vessel can eventually align itself with the new ice drift direction (ice-vane), and ice loads can reduce to the original, relatively low, levels. The problem is that pack ice may prevent the rotation of the vessel about the turret. In order for the vessel to rotate, it breaks up and clears ice upstream near the bow and downstream near the stern. This can be a slow process, and while it is happening, mooring loads may significantly increase. To mitigate such increase in the loads, faster and easier break up and clearance of ice is preferred.
A variety of vessels have been designed and/or built to deal with the particular problems associated with subsea oil and gas drilling and production in areas having significant ice incursions. One example is an FPSO (Floating Production Storage and Offloading) in Terra Nova. The Terra Nova FPSO is a turret-moored vessel equipped with a thruster-assisted position mooring system in which the thrusters are automatically controlled. However, the design of the system is primarily driven by the harsh open-water wave and wind conditions at the Terra Nova site. The pack ice design condition for Terra Nova is very mild: 5/10ths ice coverage with 0.3 m ice thickness. The Terra Nova thruster system is, thus, not designed to break up and clear ice or facilitate the ice-vaning of the vessel. The vessel has 5 thrusters (2 at the bow and 3 at the stern) arranged to optimize station-keeping performance in high wave/wind conditions. Moreover, the automatic control system for the thrusters is designed for open water conditions, and does not have any functions for determining ice drift direction or commanding the thrusters to do what is necessary to align the vessel with changing ice drift direction. The vessel is intended to disconnect and leave the field in more severe pack ice conditions, should such conditions ever occur, or in case an iceberg gets too close to it.
One typical solution includes the use of other vessels called “support icebreakers” to break up and clear the ice in the areas necessary for the moored vessel to ice-vane. This is not a satisfactory solution, as it introduces considerable operational complexity and risk. The support icebreakers have to correctly identify the prevailing ice conditions and move through the ice repeatedly to break it up and clear it. On many occasions, they will have to accomplish this in close quarters with the moored vessel and under conditions of poor visibility and other adverse weather conditions (snow, high winds, etc.). Depending on the ice conditions, more than one icebreaker may need to be active in a particular area, which increases the risk for collision between icebreakers and also between an icebreaker and the moored vessel. Because of the uncertainty about the effectiveness of icebreaker operations, this type of solution usually also includes a capability to disconnect the mooring system to avoid breaking it, if the loads due to the ice exceed the capacity of the system. While the capability to disconnect mitigates the risk of breaking mooring lines, it introduces further operational complexity and risk, particularly if the moored vessel has no propulsion and steering of its own. Failure to properly manage the vessel after disconnection may lead to collision and grounding.
Disclosed herein are several examples of vessels designed to solve some of the problems associated with sub-sea oil and gas drilling and production in arctic areas. Kvaerner Masa Yards in the 1990s developed a new type of ship for sailing in ice, named Double-Acting Tanker (DAT), which employs pulling azimuthing thrusters at the end of the ship that first meets the ice for propulsion (see K. Juurmaa, et al. infra. and U.S. Pat. No. 5,218,917). Aker-Finnyards in the 1990s built at least two multi-purpose icebreaker support vessels utilizing azimuthing thrusters, which utilize azimuthing thrusters for propulsion and maneuvering (see P. Lohi, et al. infra). Kvaerner Masa Yards in the 1990s proposed a triangular asymmetric icebreaker with three azimuthing thrusters, called the oblique icebreaker (see M. Arpiainen, et al. infra). The principle of operation is to use the entire side of the vessel to break ice, taking advantage of a special oblique hull form. By operating this way, the oblique icebreaker can break a much wider channel in the ice than ship-shape icebreakers for escorted ships to follow in. Den Norske Stats Oljeselskap has apparently developed a two-part ship for use in oil transport in arctic waters, which consists of a barge part containing a number of loading tanks and a propulsion part, which is adapted for breaking ice and has one or more azimuthing thrusters (U.S. Pat. No. 6,162,105). The propulsion part joins with the barge part for sailing through ice-covered waters (similar to a tug-barge used in open water). Upon arrival at a field location, the barge part connects to a submerged turret buoy, and the propulsion part separates from it. While the barge part is intended to ice-vane about the submerged turret buoy, it is not equipped with any active system to facilitate such ice vaning. Only the propulsion part has azimuthing thrusters.
The Canadian Marine Drilling Company (CANMAR) developed a series of ship-shape drillships, which they used for drilling operations in the Beaufort Sea. The drillships were primarily intended to drill in the open water season (summer), but to be able to withstand occasional incursions of drifting pack ice (see R. M. Hinkel, et al. infra). Frontier Drilling engaged Aker Arctic to conduct initial design and conceptual work for their turret-moored drillship Frontier Discoverer. This work includes development of a modified hull form and protection for the riser from the ice, but it does not include a special thruster arrangement or control system (see K. Bäckström infra).
Statoil and LMG Marin have developed a design for an Arctic DrillShip (ADS) with icebreaker features. The ADS has an icebreaker hull, ice cutters around the hull of the ship, thrusters aft and forward and turret mooring for water depths from 50 meters (m) to 1,000 m (see J. Jorde infra, and Int'l Patent App. WO2007/089152). However, the Statoil design requires the development of new ice cutter technology and fails to consider problems of automatic control.
At a concept level, Sandwell, Inc. conducted a paper study in 1996-97 for Mobil and Texaco, in which Sandwell developed concepts for an in-ice Floating Production, Storage and Offloading Structure (see Sandwell infra). Two of the concepts developed involved a ship-shape hull: 1) a conventional “moveable” icebreaking FPSO, which had an efficient icebreaking hull, bow thrusters for improved maneuvering, and to enhance its ice clearance and station-keeping capabilities in ice and a disconnectable mooring. This FPSO was intended to operate with ice management support of two “very capable” icebreakers, supplemented at times by a third; and 2) an extreme “permanent” FPSO, that had a much more extreme icebreaking hull with large reamers for self-ice management, with a number of azimuthing thrusters for improved maneuvering, and to enhance its ice clearance and station-keeping capabilities in ice. This FPSO was intended to rely primarily on self-ice management, but Sandwell's system included one “capable” icebreaker, supplemented at times by a second. While the mooring system was intended to be permanently connected, the concept included disconnectability in extreme situations. This concept did not include matching thruster pairs, or automatic control of the thrusters.
Also at a concept level, Kulikov and Ruksha (U.S. Pat. App. No. 2006/0096513) proposed a single-point system for tankers to moor at an offshore terminal for the purpose of loading liquids, primarily oil, from an onshore tank farm in ice conditions. This system utilizes a combination loading hose-mooring line attached to a fixed structure at the seabed allowing 360 degree)(°) rotation, and an icebreaker to lead the tanker through ice to the location of the offshore terminal, equipped with a guiding trunk that protects the loading hose from ice action. Although this system is claimed to offer “the possibility of roundabout turning,” it includes no elements specifically designed to facilitate and accelerate ice-vaning. Furthermore, this system addressed the problem of only temporary mooring of tankers in ice for a short-duration operation, with the option of stopping the operation and disconnecting the mooring.
Accordingly, an apparatus, system, and method are needed that effectively breaks up and manages ice incursions on a turret-moored marine vessel, facilitates and accelerates ice-vaning, and is capable of keeping a relative position in pack ice conditions to mitigate the impact of ice on the vessel.
Related material may be found in at least: “Global Analysis of the Terra Nova FPSO Turret Mooring System,” Paper 11914, Proceedings, Offshore Technology Conference, Houston, Tex., May 1-4, 2000; “Terra Nova Vessel Design and Construction,” Paper 11920, Proceedings, Offshore Technology Conference, Houston, Tex., May 1-4, 2000; “Experience with Drilling Operations in the US Beaufort Sea,” R. M. Hinkel, S. L. Thibodeau and A. Hippman, Paper 5685, Proceedings, Offshore Technology Conference, Houston, Texas, May 2-5, 1988; “An Arctic Drilling Campaign in Alaska,” K. Bäckström, 2nd Annual Arctic Passion Seminar, Helsinki, Finland, Mar. 15, 2007; “Arctic Drill Ship—For Year-Round Operations in Arctic Environments,” J. Jorde, 9th Annual INTSOK Conference, Houston, Tex., Mar. 27-28, 2007; “Revolutionary Oblique Icebreaker,” M. Arpiainen, M. Baeckstroem and R-A. Suojanen, Proceedings, POAC 1999, Helsinki, Finland, August 23-27, 1999; “Mobil/Texaco In-Ice Floating Production, Storage and Offloading Structure Feasibility Study,” report by Sandwell, Inc., Vancouver, BC, Canada, August 1997; “New Ice Breaking Tanker Concept for the Arctic (DAT),” K. Juurmaa, G. Wilkman and M. Baeckstroem, Proceedings, 13th International Conference on Port and Ocean Engineering under Arctic Conditions (POAC), Murmansk, Russia, Aug. 15-18, 1995; “MSV Fennica, A Novel Icebreaker Concept,” P. Lohi, H. Soininen and A. Keinonen, Proceedings, IceTech '94, Calgary, Alberta, Canada, 1994; Int'l Patent App. WO2007/089152; U.S. Patent App. 2006/0096513; U.S. Pat. Nos. 6,848,382; 6,799,528; 6,162,105; 5,218,917; and 4,747,359.